The present invention relates generally to adhesive additives for paper manufacturing. More particularly, it concerns a suspension-enhancing, relatively nontoxic adhesive additive for making corrugated containers of paperboard.
The general manufacturing techniques for making corrugated containers of paperboard, also referred to in the industry simply as corrugated paperboard, are well known. As background for the present invention, the disclosures of U.S. Pat. No. 4,853,072 to Thompson and U.S. Pat. No. 4,424,291 to Leake, et al. are incorporated herein by reference.
Essentially, corrugated paperboard is made from two types of components, a corrugated paper one and plural planar (i.e. uncorrugated) paper ones. Corrugated paperboard is assembled by sandwiching the corrugated paper component between opposing planar paper components. In the context of sandwiching those components, it is necessary to make a bond between inner surfaces of the opposing planar paper components and outer surfaces of the corrugated paper component.
To make such a bond, various adhesives have been proposed, and most of them are based on an aqueous mixture of starch and certain additives that impart desired qualities. For example, it is known to make the adhesive water-resistant by adding an effective amount of a urea-formaldehyde polymer resin. Other proposals include adding surfactants to increase the permeability of the paperboard, adding defoaming agents, and adding biocides. As used herein, starch means any suitable starch product such as cornstarch, or any other known starch, examples of which are described in Leake et al.
To prepare such adhesives :for application to desired surfaces of the planar and corrugated paper components, conventional methods involve making a so-called "starch batch" of adhesive according to the following steps:
(1) preparing a "pre-gel" or so-called carrier starch by combining an effective amount of a special starch to an NaOH solution and heating the resulting mixture to about 140.degree.-145.degree. F., thereby causing it to become a thixotropic paste-like substance; PA1 (2) adding a desired additive like a urea-formaldehyde polymer resin; PA1 (3) diluting the "pre-gel"/carrier starch with a desired amount of water; and PA1 (4) adding an additional, desired amount of ordinary starch.
By "special starch", applicant means a starch characterized chemically by having a highly branched configuration. By "ordinary starch", applicant means a starch characterized chemically by having a straight-chain configuration. Both types of starch are commercially available, but there is a higher cost associated with the special kind.
By preparing the "pre-gel"/carrier starch, conventional adhesives are capable of keeping the ordinary starch in suspension. To use ultimately the conventional "starch batch" of adhesive after following the above preparation process, the starch batch is delivered to a suitable applicator machine which heats it to about 350.degree. F. and applies it to desired surfaces of the planar and corrugated paper components. Such known machinery is shown and an example is described in. Thompson. The machinery is generally referred to as a corrugating machine, or corrugator.
Referring again to conventional adhesive proposals, there have been attempts to modify the aqueous-starch-adhesive mixture with various synthetic polymer resins such as emulsion polymers, solution polymers, acid-curable polymers, and alkaline-curable polymers. Each is designed to provide certain improved features. For example, Leake, et al. describes a certain acrylic-acid-based adhesive composition that is designed to improve tack performance. Tack, also referred to in the industry as "green bond strength", is the bond strength between the planar paper components and the corrugated paper component immediately after their adhesive-wetted surfaces are contacted.
None of the conventional proposals for such adhesive compositions has focused on providing enhanced suspension without using a special starch. Conventional "pre-gel"/carrier starches are always prepared using a special starch. Also lacking in the prior art is an adhesive composition that provides a combination of increased runnability and water resistance, while at the same time providing an end product that dries harder than that made with conventional compositions. The feature of drying harder would allow for a reduction in thickness of paperboard used in such manufacturing, and a corresponding reduction in raw-material costs. The resulting harder paperboard product would also respond to printing better than that made from conventional compositions.
Accordingly, it is a principal object of the present invention to provide an adhesive additive for starch-containing adhesives used to manufacture corrugated paperboard that overcomes the drawbacks and limitations of prior art proposals.
Another object is to provide such an additive that enhances suspension of such starch without having to use a special starch characterized chemically by having a highly branched configuration.
Yet another object is to provide an adhesive based on such an additive that offers a combination of increased runnability and water resistance, while also providing a finished paperboard product that dries harder than that made with conventional compositions.
Another important object of the invention is to provide a method for preparing such an adhesive based on such an additive.
It is also an object of the invention to provide such an adhesive additive that can be easily and cost-effectively manufactured.
A further object of the invention is to provide such an adhesive that lacks undesired, highly toxic resins like urea-formaldehyde resin.
In brief summary, one aspect of the invention includes a suspension-enhancing adhesive additive for an aqueous, starch-containing, liquid adhesive to be used in manufacturing corrugated paperboard. The additive includes a suspension-enhancing agent with a synthetic liquid polymer component, and which polymer component is emulsifiable in the adhesive, and is capable of enhancing suspension of such starch. The polymer component is based on at least one acrylic-acid monomer, and may be a copolymer based on an acrylic-acid monomer and an aromatic monomer substituted with an acrylate compound.
Another aspect of the invention involves a liquid adhesive composition for manufacturing corrugated paperboard. That composition includes an effective amount of a starch whose chemical composition is characterized by having a substantially straight-chain configuration. Also included is a suspension-enhancing agent like that described above. An amount of water is also included so that the overall composition includes between 0.5%-1.0% by weight of the additive, between 21.5-25.5% by weight of the starch, and between 73.5-78% by weight of the water.
Another aspect of the invention is a method of preparing a starch-based, liquid adhesive composition for manufacturing corrugated paperboard. The method includes the steps of (1) using an effective amount of a starch, characterized by having a substantially straight-chain configuration, to make a starch gel, (2) adding an additional, desired amount of the starch, (3) enhancing suspension of starch by adding to the composition a synthetic liquid polymer component capable of being emulsified in the composition, and (4) diluting the starch gel with a desired amount of water.
These and additional objects and advantages of the present invention will be more readily understood after a consideration of the drawings and the detailed description of the preferred embodiment.